The present invention relates to pressurized water nuclear reactors (PWRs). More particularly, the present invention relates to reduced in-core instrumentation patterns for a pressurized water nuclear reactor.
Pressurized water nuclear reactors are equipped with installations making for periodically and remotely measuring the neutron flux at certain points of the reactor core. Measuring instruments and sensors are installed in nuclear plants for the measurement of different types of radiation primarily neutrons and gamma rays). In addition, non-nuclear instrumentation is installed to measure process parameters, such as temperature, pressure, flow, and so on. Each instrument is part of a channel that comprises the sensor, a signal transmission line, amplifier, or other electronics, and meters, indicators, or recorders at the other end. The measured parameters can be channeled, according to type and importance, in different ways.
The power production level in a PWR is monitored through two instrumentation systems: the ex-core and the in-core instruments. The ex-core instruments measure gross neutron flux and hence total power level, whereas the in-core instruments measure local power levels. All ex-core neutron detectors are located in groups outside the reactor vessel, typically adjacent the inner side of the shielding wall. Each group of neutron detectors incorporates more than one unit to provide redundancy as mandated by regulatory requirements. These instruments are used at reactor start-up, when control rods are slowly withdrawn from the core, to determine neutron rate increases. Beside providing a neutron flux level indication, the signal is also fed to an electronic differentiating circuit that calculates the rate of change of neutron flux.
In-core instrumentation is used in PWRs to provide a more detailed picture of power levels inside the core. The local power density in nuclear reactors is often measured by the use of a plurality of these in-core detectors, each of which is contained in an elongated guide tube which guides the instrument through a nuclear fuel assembly. Together, the instrument and guide tube are typically called an in-core instrument or instrument assembly. The in-core instruments (ICI) are exposed to very high radiation levels and therefore may become very highly radioactive. This radioactivity makes the ICI tube and instrument extremely dangerous to handle when exhausted instruments are to be disposed of, usually during a reactor refueling outage. In-core instruments are also utilized in conventional PWRs for monitoring and surveillance functions on core power peaks and core power tilts, as well as for the detection of fuel misloadings. Specifically, the ICIs must permit the determination of core power peaks and core power tilts through each operating cycle such that any uncertainties in these calculated quantities are within limits that have been licensed for the plant by the United States Nuclear Regulatory Commission (USNRC). Also, the ICIs must enable the detection of misloading of any fuel assembly in the core at the beginning of each cycle of operation.
As noted above, the instrument is contained within a guide tube within the core. In order to avoid disturbing the core excessively, small channels and instruments must be used, usually of the self-powered very thin type. Various means of distributing these instruments throughout the core are known. In typical PWR cores, about 25% of the assemblies in the core incorporate instrument channels in which the instrument is inserted. Moreover, lead wire connections must be made to the instruments through various penetrations provided, either through the bottom or the top of the reactor vessel. The removal and transfer of exhausted ICIs during refueling is performed entirely under a sufficient depth of water to make use of the radiation shielding effect of the water. This requirement, however, often puts the ICI removal activities on the critical path during reactor refueling, especially in reactor installation where the ICIs enter the core through the top of the reactor vessel. Often, the only place in the reactor installation where sufficient water depth exists is directly over the reactor. Thus, the major refueling operations cannot be performed until the ICI replacement operation is completed. During a typical refueling, twenty to thirty ICIs must be individually removed and disposed of.
Finally, current ICI patterns include a significant level of redundancy to ensure that the ICIs can be used to carry out their intended functions in the event of unexpected ICI failures.
From the foregoing description, it can be appreciated that significant savings in refueling time and costs may be achieved if the number of ICIs is reduced.
It is an object of the present invention to provide a method for reducing the number of in-core instruments (ICIs) used in pressurized water reactors. It is a further object of this invention to reduce the number of ICIs while preserving the capability of the ICIs to perform all necessary functions.
It is another object of the invention to reduce the number of ICIs in operating plants to achieve initial capital and life cycle savings by reducing outage time, the amount of equipment necessary to support the ICI system, and the number of ICIs that would need to be replaced during the life of the plant.
According to the present invention, a method for determining a reduced ICI pattern is provided based upon the considerations recommended by the USNRC for inclusion in and evaluation of changes to the ICI system made in accordance with appropriate federal regulations (10 C.F.R. 50.59). According to the inventive method, candidate ICI patterns having a reduced number of ICIs relative to the existing pattern are first selected. After selection, the candidate patterns are evaluated to ensure that any differences between the predicted core power distributions and those synthesized from the reduced ICI patterns are in compliance with the limits that have been licensed for a particular PWR. The candidate patterns are also evaluated to ensure that the reduced number of ICIs provides the capability to detect misloading of a fuel assembly into an improper location. Finally, the candidate ICI patterns are evaluated to ensure that the reduced ICI patterns are still functional within the current Technical Specification ICI operability limit. Currently, the Technical Specification requires full function of the ICIs when only 75% of the ICIs are operable, in accordance with plant requirements.
Reducing the number of ICIs that must be replaced every few cycles of operation leads to a reduction in the number of ICIs that would need to be replaced over the life of a plant. As a result, significant savings could be achieved in life cycle costs for the ICI system, resulting from savings in both hardware replacement costs and hardware disposal costs. In addition, because fewer ICIs would have to be replaced during each refueling outage, savings in plant outage time and reductions in radiation exposure to plant personnel are also achieved. Finally, reducing the number of ICIs in plants that have not yet been built would provide additional reductions in the equipment needed to support the ICI system. As a result, significant reductions in overall capital costs for new PWRs as well as reduced operating and maintenance costs for these plants could be achieved.